The way a particular drug is administered to a recipient can significantly affect the efficacy of the drug. For example, some therapies, in order to be optimal, require that the drug be administered locally to a particular target site. Furthermore, some of those drugs need to be present at the target site for a prolonged period of time to exert maximal effect.
One approach for achieving localized drug delivery involves the injection of drug directly into the site of desired drug activity. Unfortunately, this approach may require periodic injections of drug to maintain an effective drug concentration at the target site. In order to prolong the existence at the target site, the drug may be formulated into a slow release formulation (see, for example, Langer (1998) NATURE 392, Supp. 5-10). For example, the drug can be conjugated with polymers which, when administered to an individual, are then degraded, for example, by proteolytic enzymes or by hydrolysis, to gradually release drug into the target site. Similarly, drug can be trapped throughout insoluble matrices. Following administration, drug then is released via diffusion out of, or via erosion of the matrices. Alternatively, drug can be encapsulated within a semi-permeable membrane or liposome. Following administration, the drug is released either by diffusion through the membrane or via breakdown of the membrane. However, problems associated with localized drug injection can include, for example, repeated visits to a health care professional for repeated injections, difficulty in stabilizing drugs within slow release formulations, and the control of the concentration profile of the drug over time at the target site.
Another approach for localized drug delivery includes the insertion of a catheter to direct the drug to the desired target location. The drug can be pushed along the catheter from a drug reservoir to the target site via, for example, a pump or gravity feed. Typically, this approach employs an extracorporeal pump, an extracorporeal drug reservoir, or both an extracorporeal pump and extracorporeal drug reservoir. Disadvantages can include, for example, the risk of infection at the catheter's point of entry into the recipient's body, and that because of their size the pump and/or the reservoir may compromise the mobility and life style of the recipient.
Over the years, implantable drug delivery devices have been developed to address some of the disadvantages associated with localized injection of drug or the catheter-based procedures. A variety of implantable drug delivery devices have been developed to date.
One type of implantable drug delivery device includes the osmotically driven device. A variety of osmotic drug delivery devices are known in the art. For example, one such device is available commercially from Durect Corp. (Cupertino, Calif.) under the tradename DUROS®. Similarly another device is available from ALZA Scientific Products (Mountain View, Calif.), under the tradename ALZET®. In some devices, the influx of fluid into the device causes an osmotically active agent to swell. The swelling action can then be employed to push drug initially stored in a reservoir out of the device. DUROS® pumps reportedly deliver up to 200 mg of drug at rates as low as 0.5 μL per day. However, osmotic pumps stop working when the osmotic engine in the device or drug reservoir becomes exhausted. A variety of different osmotically driven drug delivery devices are described, for example, in U.S. Pat. Nos. 4,957,494, 5,236,689 and 5,391,381.
In addition to osmotically driven drug delivery devices, a variety of mechanical and electrochemical devices have been developed to date. U.S. Pat. No. 3,692,027 describes an implantable, electromechanical drug delivery device. The device includes, within a fluid impermeable and sealed casing, a watch-type drive mechanism that drives a circular wheel. The wheel contains a plurality of cavities, all of which apparently are radially disposed in a single diametral plane about the circumference of the wheel. Once the drug-containing cavity moves into alignment with an aperture through the casing, a piston associated with the cavity ejects medicine out of the cavity and through the aperture. This type of device can be quite large in size and, therefore, may be unsuitable for implantation into small cavities within the body.
U.S. Pat. No. 6,283,949 B1 discloses an implantable drug delivery device that includes a reservoir, a dispensing chamber adjacent to the reservoir, a dispensing passage provided along an interior surface of the dispensing chamber, and an actuator for applying a moving compressive force onto the dispensing passage. As a compressive force applied by the actuator moves along the dispensing passage, drug is simultaneously ejected out of the dispensing passage into a catheter for delivery to the target site, and additional drug is drawn into the dispensing passage from the reservoir. The size of this type of device may limit its applicability when implantation into a small body cavity is desired.
U.S. Pat. Nos. 5,797,898 and 6,123,861 disclose microchip based drug delivery devices. A plurality of drug reservoirs are etched into a substrate, for example, a single microchip. Drugs then are sealed within each of the reservoirs with a seal. The seal can be either a material that degrades over time or a material that dissolves upon application of an electric potential. See also Santini et al. (1999) NATURE 397: 335-338, which similarly discloses a solid-state silicon microchip that provides controlled release of a drug of interest via electrochemical dissolution of a thin membrane covering a micro-reservoir filled with drug.
However, there is still an ongoing need in the art for reliable, miniaturized, implantable drug delivery devices that permit the localized delivery of a drug of interest over a prolonged period of time.